1. Field of the Invention
This invention relates to a photochromic lens equipped, on at least one side thereof, with a photochromic layer formed of a resin which contains a spirooxazine compound as a photochromic substance.
2. Description of the Prior Art
Photochromism is a phenomenon of reversible changes in color, which take place when compounds of a certain sort are exposed to sunlight or ultraviolet-containing light such as light emitted from a mercury vapor lamp and after such sunlight or ultraviolet-containing light is removed. Lenses making use of the above-mentioned function of such compounds are widely known as photochromic lenses. They have been finding wide-spread commercial utility in sunglasses, prescription eyeglasses, goggles and so on.
Lenses having such a function have heretofore been limited to those obtained by dispersing minute particles of a silver halide in lenses made of inorganic glass. Almost no resin-made photochromic lenses have however been employed yet on any commercial scale, although more and more resin-made lenses have been used these days.
The following reasons may be mentioned for the lack of resin-made photochromic lenses. An inorganic material such as silver halide particles has low compatibility with resins so that it is impossible to disperse them uniformly to any sufficiently high concentration in the resins. Although organic photochromic compounds appear to have high compatibility with resins, there are only few organic photochromic compounds which are stable and useful.
For example, spiropyran compounds which are most common organic photochromic compounds exhibit useful photochromism as long as they remain in an organic solvent. Their fading rates, namely, the rates at which they regain their original colors are extremely slow in high molecular materials employed widely as optical materials and having relatively high second-order transition temperatures. For this reason, significant limitations are imposed in application on the resulting photochromic materials.
On the other hand, Japanese Patent Publication No. 28892/1970 discloses that spirooxazine compounds show photochromic effects in usual non-crosslinked polymers no matter what second-order transition temperatures the polymers have. As pointed out in Japanese Patent Publication No. 48631/1974, these spirooxazine compounds are however, accompanied by a drawback that they are not colored to any substantial extents at high temperatures above room temperature due to their high fading rates at such temperatures although they change very well in color, namely, they are colored and faded significantly upon exposure to light and removal of the light at low temperatures below 10.degree. C. Since a photochromic lens is generally intended to develop a color or to changes its color upon exposure to strong light radiated from the sun so as to reduce the transmission of the light, it is obviously required to have sufficient coloring effect in environments of temperatures somewhat higher than normal temperature. It is here that certain limitations are generally imposed on the use of such spirooxazine compounds as photochromic materials for photochromic lenses.